Maybe you pressed the insert key, which will change the Eclipse editor to Overwrite rather than Smart Insert (see the info bar at the bottom of the editor), and will change the cursor to a block rather than a vertical line?
Update: Thanks for clarification - see mikej's answer which is correct. I'll leave this answer in case anyone has the similar, related problem that I describe.
It sounds like you have accidentally switched from Smart Insert mode to Insert mode.
Press Ctrl + Shift + Insert or tick Smart Insert Mode on the Edit menu to switch back.
Smart Insert is the feature that automatically insert closing quotes and brackets when you type the opener and places semi-colons at the end of the line when you press semi-colon (if you have the preference for that enabled.)
@Jeremy by saying " I have a vertical line with a gap in the middle of it, like the character above the forward slash " i guess you are referring to the ' Pipeline 'symbol, the one you use to denote' OR ' in programming languages. As people already answered, you are in Raw Insert Mode (as opposed to Smart Mode) so try Edit->Smart Insert Mode (Ctrl+Shift+Insert by default).
For more info you can visit this thread http://www.eclipse.org/forums/index.php/t/53833/
And remember, the symbol is called 'Pipe' symbol or 'Pipeline' symbol.
