Question Pc doesn't work

[Ivkee1231]

Okay so a friend gave me his pc to put a new windows on it and to clean it a bit. Thats what I did, put on new cooling paste and stuff but the Pc wont turn on now. It turned on and prompted me to 'reboot the system and choose a valid bootable drive' and I retarted the pc a couple of times trying to se what the problem was, now it wont turn on at all. It turns on but there is nothing on the monitor and the lights on the front are not flashing anymore. Can someone help me diagnose this? Thanks in advance

 

[Grobe]

Full HW speccs and relavant history of it?

 

[Ivkee1231]

Full HW speccs and relavant history of it?

Ugh, it's an old pc, nothing special. Was just sitting in his room collecting dust basically