auggie1020,
It would be great if you can find the old cover and use two SS screws to attach it back. If the old cover is broken or not available then there are replacements that will fit into the existing ring. Usually, calling a plaster contractor or builder is worth the service call because it's going to be hard to hold your breath long enough to install.
Definitely needs to be addressed before using pool. If your drain line has a separate valve in front of the pump, it should be turned off until repairs are made.

Just to echo golfgeek, it is a very dangerous situation currently. No one should be near it if the pump is running.

The cover you had on it should be VGB approved, as is whatever new one you put on it.

They come in different shapes and sizes. The drain fixture (the part that the cover is screwed to, should have a manufacturer name and a model number on it. (probably raised letters in the same PVC material as the fixture) With that info, you can search for a replacement cover.
Around here, you need a double main drain, with at least 48" separation and approved covers, to preclude accidental entrapment.

Our main drain cover was missing and our pool guy wouldn't even continue to provide us service without having it fixed. He said the situation was so dangerous that it would render his insurance null and void.

In order to have it fixed, a person with a diving suit had to come out and do it. It takes a little longer than just holding your breath. No worries though, if I remember correctly, the cost was less than $100.