[smju]

Unless you were editing the php files it is highly unlikely {if not impossible} that what you were doing messed one or more of them up.

And if you were editing php files, revert means to replace the files you edited with the backup copy you made before doing the edits.

First though if I were you I'd follow MarcoH64's advice and pop along to templates find the NAVBAR one and click revert.

See if that solves your problem.

If it doesn't I suspect that you will need to provide a lot more info about what you were doing prior to this problem manifesting itself.