Getting 504 Error when trying to add images to page

A few weeks ago, I deleted A LOT of images off my site that were not needed and were taking up too much space. I used FTP because there were 100’s of images and I thought it would be faster. I would delete the images and then look in Media to make sure they were not there – which they weren’t. I noticed yesterday that there was a problem. I was creating a page and I wanted to insert an image. I clicked the button to insert an image and when I was scrolling through the images, I noticed that there were several blank squares with the broken image icon in the corner. I clicked on one of those images, and the site lost connection and I got a 503 Service Unavailable error. I had to close out all the windows and then I could open a new window and my site would come back. I look at the error log, and at the moment I clicked on the broken image, there is a large load on the site. I then went to the media section to see if I could see the blank images and they are not there. So the images I deleted via FTP are hung up within the “insert media” function within a page or post, but not in the media section. How do I clear this out so it works?