Screw hole metal ring has popped off

It sounds like a bushing, brass with threads for the screws. The plastic mount for it has probably cracked, the real fix is to replace the case part with the broken plastic mount. But, there are two ways to deal with it. First you can ignore the missing screw if the case is firmly held together, if the case shows a gap then try the second method. The second method is not guaranteed to work but stands a chance to work, use super glue or better use epoxy to remount the bushing into the plastic mount and hopefully fix the cracks in that as well. When gluing the bushing, take care to keep glue out of the threaded area and also wait for the glue to cure. If that fails then let us know where the bushing came out.