Help! Car Wont Do Anything!

Hi, first post here! So i moved to america and decided to get me a mustang to go to school and as a project, and i bought a 98 3.8 5 speed for just 800$ in very good conditions!! It was sitting for about 2 years when i bought it and i got it runing with just replacing the fuel pump, battrry, and fluids and all that stuff.... It was running like a champ until one day it decided not to do anything.... I put the key on and it wont crank and not a single light works, just nothing works... Like if it had no battery, it will start with jump but as soon as i turn it off it wont do anything, nothing electrical will work,I cheked all the fuses and they are all fine, what do you guys think? Any info will be really appreciated thanks!

Sorry for the long time... It turned out to be a bad battery, even though it was brand new. Now that means the car kills batteries... Any ideas of what might be causing that? Also someone attempted to steal the car last night ( good thing battery was dead haha) and know the key will not open the door because the door lock is really damaged, and to make things worst i dont have a remote or FOB to open it... Is it fixabe without having to use a different key on that door than the others? Thanks a lot!