The KB article attached by @battybishop explains it all.
The new versions of ESXi(4.x, 5.x, 6.x) they do not support resetting password in case you have forgotten, in which case you will have to reinstall the ESXi host (this is the only supported way).
In case if this was a scripted installation / Auto deploy based installation, you should check your script / host profile on vCenter Server to find out the password.
Hope it helps.
Regards
Ravi Verma