Hello! Last week i borrowed my friend's Arduino Leonardo and started messing with it, it all worked just fine untill today. When i connect it to my pc it doesn't show up anywhere, and when i power it up the power and led lights stay on forever. Could you please help me with my problem? Is the board dead? Did this happened because i uninstalled the arduino drivers while the board was still plugged in the pc? I have to give it back in 2 days and i don't know what to do 
Uninstalling the drivers while it was plugged in can not destroy it.
I think the board is dead. But it can happen, it might not be your fault at all. It might have to do with static electricity.
Other things that could destroy a board are inductive motors or relays or other coils, shortcuts, loose wires with 12V, batteries with a voltage above 5V, and so on.
Does your friend have a programmer ? With a programmer it is possible to burn the bootloader. Sometimes that helps.
Let your friend read this post. Arduino is about testing and prototyping and learning while doing things. As a result a board can be destroyed. It happens.
Is it an official board of 18...21 euros / 23...28 dollars ? Or a clone from Ebay for 10 dollars ?
If it is a clone, perhaps it has bad quality components that failed.
Thank you for the info and for the fast response, he bought it 2 weeks ago with 25 euros, it's an official board, and the weird thing is that, this is the first time powering the arduino after i uninstalled the drivers.
I think that is coincidence (I'm 80% sure of it).
Thank you for all the help, i tried connecting my board and now it works, don't know what happened there, maybe it's the same problem as his: Periodically, Mega com port isnt' detected by arduino IDE? - Installation & Troubleshooting - Arduino Forum . Anyway, the good thing is that now it works and everything is ok, thanks again.
So I was 80% wrong ? 
I'm glad you have it working, and it is not broken. 8)
now, check e-bay and get your own board. better to mess up your own board than your friends !