Mouse moving but iMac frozen and missing icons

Hi, I have 2007 iMac I got a message that my start up memory is very low, so I tried to move some files from the Mac to my back up storage device and I accidentally dropped files on the desktop and not into the storage icon. As a result imac froze, I force quit it by pushing the power button and it came back up but ever since the mouse is moving but the iMac is not responsive. Nothing works, I can’t see any icons I had saved on the desktop and it doesn’t recognize anything connected via USB port. Is there anything I can do to fix it? I presume I jammed the memory with that accidentally copying command. What do I do?