Toyota Hilux d/cabin no fwd light not showing,its engaging but has no power,what could be the problem.

Go here so you can find out how to replace and or clean the filter .
https://www.youtube.com/watch?v=8zFIRHp-NS courtesy of youtube . https://www.youtube.com/watch?v=8zFIRHp-NS

Has the Transfer case fluid ever been replaced/changed? If not do so it may be sticking from dirty fluid.

Check this out, is for a 08 hilux but should be the same...

Toyota Hilux engine power loss Questions & Answers - Fixya

www.fixya.com > ... > Tags > engine power loss > Cars & Trucks > Toyota
Toyota Hilux cars & trucks engine power loss questions and answers. Ask your Toyota engine power loss related question. ... Question about 2006 Hilux 3.0.

check power to brake light switch. if no power then try to trouble shoot and restore power to brake light pedal switch

FSM time
ever heard of the FSM. its a magic book that tells you how to do the diagnosis of all parts on car.
get one.
alldata.com is a close subs.

Based on what you wrote about sitting for a week, my first thought is the battery. Does the truck start?

Next, do you get an error code with OBD2? I have a 97 4Runner that had a similar problem shifting into and out of overdrive. It was an electrical relay inside the tranny.

My thought is that if the battery is low perhaps a similar relay is trying to engage or disengage 4x4. Or that relay is just bad.

Could also be the 4X4 switch itself.

check the plugs on the front diff or on the back of the transfur case there will be a solenoid in one of these plases and it may need freeing up

Have you tried putting it into neutral then chucking it into low ratio?