Ok, check 'Control Panel > Add or Remove programs', or whatever it is called in 98. Is it there?
If not; something has caused it to be deleted or get uninstalled, therefore you cannot just 'get back' your media player you must install it again.
You could also try searching your hard drive for "WMP" or "Windows Media" or "Media Player", try "Media" or "Player" by themselves if you're not getting anywhere. Some of them should get some hits if indeed there is a trace of the program still.
You didn't answer my question about what you think may have caused it.