My computer wont start unless I remove my HD 5770 Powercolor

Don't put your card in the oven...

Did you remove the heatsink?

Is the card still under warranty?

Have you tried reseating it?

Have you got the PCI-E cable plugged in?

What did you clean it with?

Have you tried it in another PC?

How did you clean it? Why you want to put it in an over?

If the temperatures go above 85-120C, your capacitors might blast. Some eccentric people have actually done this kind of "Oven Experiments" on electronics. Google what is their results  

Did you remove the heatsink?
yes I did  

Is the card still under warranty?
yes it is but its useless in my country

Have you tried reseating it?
I dont know what do you mean

Have you got the PCI-E cable plugged in?
yes I've done everything right as I think  

What did you clean it with?
first I cleaned it with a Brush teeth to remove the dust but there was a sticky Dust so I removed it by a negative Screw  

Have you tried it in another PC?
Yes, I have and it didn't work

How did you clean it? Why you want to put it in an over?
because I red that it has worked with many people  

Its not recommended to remove the heatsink if the card is still in warranty as it voids the warranty...

By "reseating" I mean re-installing the card in the motherboards PCI-E slot, but as you've tested it in another PC its unlikely to be the cause of the issue.

Don't use a tooth brush to clean a graphics card (or any PC component) its far to rough and could easily damage the card, also if the tooth brush had been used before it may have had some liquid/moisture on it.

I think you've killed it by ESD or damaging the PCB when cleaning.

Next time use a can of compressed air.

Next time use a can of compressed air
are you kidding ??  
I killed myself to get this card
I should learn from this mistake  
thanks anyway (there is no any hope right?   )

Nope im not kidding, compressed air works well and wont damage your components

If you've tested it in another PC with a power supply that has ample power then the card is almost certainly dead.

No No I'm giving you a Comment for the word "Next time" Not "use a can of compressed air "

Did you reapply thermal paste between heatsink and GPU?

yes but it didn't work also
so I removed it again  

Thermal paste is needed to fill in any imperfections in the surface of the GPU and heatsink, you need a small amount (roughly the size of a grain of uncooked rice). It aids in heat transfer...

its useless I tried it