I just installed Mojave on my MacBookAir, and now my username/password (under Yosemite) are not accepted. I called Apple support and they told me to restart and immediately push down command-r giving me a MacOSUtilities window. I, then, chose the DiskUtility option that was offered, but the advisor didn't know what further to do, and scheduled me for a further call tomorrow.
Any immediate advice (somewhat disappointing--one would seem to expect smoother installation behavior from Apple)? Perhaps I'll have to take it to an Apple store for assistance.
Do I need the original "admin password"? (The laptop was purchased several years ago. I don't know if offhand.)