My computer won't shut down or restart.

I have an iMac. The trash folder won't open, but instead remains highlighted and nothing else can be opened. Eventually it returns to normal after about a minute and a half. Also, the computer won't restart or shut down. When I've tried, absolutely nothing happens on screen, and pressing the power button at the back just sleeps the computer. These problems started last night, and seem to be related, but I have no idea what to do. I have very little computer expertise.

 

Okay, as dumb as I feel right now, I'm glad it was a ridiculously simple fix. Everything's back to normal. I don't understand why that happened, but thank you for your help.