Issues with my desktop PC over the last week

Last week after a power outtage, my PC just wouldn't turn on, there were no lights inside, so I assumed that the PSU probably had issues so yesterday I went to the guy I got the PSU from to check the computer out and replace the PSU if needed, now, the he found out that the some cables inside my pc werent connected( were probably loose before that because the friend who helped me upgrade the pc a few months back didn't do a spectacular job connecting everything) so he reconnected cable l, and the pc worked again.

Today, after using the pc, then sleeping a few hours leaving it on, when I woke up, the monitors were getting no signal although the pc was on, and there were lights inside, so I held the power button for a few minutes to close the computer so I could restart it, but the pc wouldn't shut down for some reason, so I went to the power switch behind the pc, switched it off, the pc shut off, then switched it on, but before I could press the power button on the front, the pc turned itself on, but still the monitors were getting no signal and holding the power button still didn't work, so I both turned off the power switch behind the pc, and took off the power cable, and then when I reconnected the cable and turned on the power switch,and now the computer just won't turn on using the power button, so now im back to the issue I had until yesterday, and I can't come back to the guy who helped me solve last problem until sunday, so I wanted to ask if there is anything I can do to try to fix this issue by myself so I won't need to wait to sunday and take my whole pc there? I'm thinking this issue might be related to the power button in the front, but i'm no expert which is why i'm asking you guys here for help, maybe from this story thus far you can figure out a reason for this issue.