AE-II FILM SPEED DIAL LOCK. THE LOCK BUTTON REMAINS DEPRESSED SO THAT THE FILM SPEED DIAL IS NOT LOCKED

What manufacturer and model camera is it, please?

In manual mode you should be able to change the shutter speed simply by turning the main dial (the thumbwheel located near the shutter release button).

Canon AE-1 35mm SLR is a manual focus camera EOS is Canon's designation of auto focus EOS stands for Electro-Optical System. The problem you are experiencing is caused by several little things that throws the camera, wind and cocking leaver out of synchronization or simply come to the end of the roll. There is a frame counter window just to the left of the shutter release behind the shutter speed dial. Also if you have just loaded film in the camera it may not be seated correctly check by releasing the rewind button, wind the film back but not all the way and open the camera to restart the film.

Diagram

That can be a problem. The ASA dial has a post that links to another ring under the top cover. The ring under the cover is spring loaded via a tungsten cable that travels the span of the top of the camera to the rewind side. If you are not careful, the cable could break.

To get to the dial, unscrew the cap screw that holds the film advance lever on. The cap screw has 2 small holes for a specific tool, but small needle nose pliers with fine points will work. The lever can be removed with 2 washers along with it. You can now see the retaining nut for the dial. It is best to set the dial at 'B', 3200 ASA. Use the same pliers to remove the nut. The dial can be removed at this point. Best to lift it on the far side near the edge of the top. If the dial itself is damaged, I probably have a good used one. The ring under the top has a notch for the ASA dial post, at rest it should be at about 10:00 oclock. With the ASA dial still set at B/3200, lean the post into the notch and slowly turn it counter-clockwise till the ASA dial can be seated fully. Reassemble the other parts in the reverse order.

every time you change lenses if you are using a wide you have to lock shutter mirror back first

You should have a small chrome button near the film speed dial. Press & hold it down while turning the film speed dial.

Set the film speed with the dial on top of the Photomic unit. Lift the chrome ring and turn so that the film speed is next to the little red arrow.
Yes, the aperture is set by the lens ring. And if you have one of the older finders which uses the prong meter connection, whenever you mount a lens you must turn the aperture ring all the way to the highest number, then back to the lowest number. For old-timers, this is known as the "Nikon twist". It tells the meter system what the widest aperture of the lens is so that it can measure light properly at full aperture.
Yes, the shutter speed is set by the dial on top. The Photomic meter sits on top of it, so you read the shutter speed from the scale on the side of the Photomic dial by the mark on the back.
The other buttons on the front are the lens release button (left side as you hold the camera), DOF preview (button just under shutter release), mirror lock-up (lever ring around DOF button) and self-timer (long lever at bottom).
The T-L ring serves to protect the shutter release from accidental push. If you lift the ring and turn the mark to L, the shutter release is Locked. If you lift and turn to T, it prepares the shutter for Time operation, where the shutter remains open when released without holding the button down as you do at the B speed setting.

Finally, the most help: you can download the manual for free (donation requested, and worth it) at http://www.butkus.org/chinon/nikon/nikon_f2_photomic/nikon_f2_photomic.htm

It sounds like you have a jam.  When you have finished taking shots and have used all the film, you must press the film-rewind button on the underside of the camera; this does not rewind the film, but allows you to commence the next step.  Once you have done this, flip up the rewind-crank on the top left hand side of the camera. It is situated on top of the film speed dial. turn this clockwise until all the film is back in its cartridge.  You should now be able to open the back of the camera.
I hope you have fun with your t50!

I'm afraid you have a somewhat serious problem. There is a tungsten cable that runs from the speed dial to the rewind side if the camera. This cable sets the brushes for the electrical contacts the are under the rewind assembly. Adjusting speeds or ASA changes the position of the contacts.

It is very likely that the cable has broken and needs replaced. Canon no longer supports these nor do they have parts

To open the back pull on the rewind knob ( pulling up ) until back opens. Check that your 6 volt battery is ok ( ae -1 has an electronic shutter so will not work without a battery ): battery situated on the left hand side of the camera ( if it is facing you ) underneath small flip up door ( lock is at the bottom on the left of the small door ) . Check the battery by pressing the shutter release gently and seeing if your light meter needle moves. No movement ? replace battery. You have two modes for taking pictures : auto which is actually AV ( aperture priority) or full manual : both are set by moving the shutter speed dial on the top right hand side ( underneath film winding lever ).