Locked out of switch

Due to the problem I had on my switches (2950s) recently I decided to wipe
them both & start afresh, this worked fine with SWITCH1, but unfortunately
after initially configuring SWITCH2 I somehow have locked myself out of
both the console login & the vty login. At both prompts I am asked for a
username, but whatever I type in I get the message:

% Login invalid

I am sure I set everything up as previously, but even allowing for spelling
mistakes and capitals I have tried every possible combination to no avail.
Could I have set up to prompt for a username & not set one? Either way,
does anyone know how I can reset the switch without access to exec mode?

Any suggestions greatly received, TIA, Jason