Please provide additional information:
1. When you say Hosted 3CX, do you mean your 3CX is hosted at Amazon, Google, or 3CX? Where is it hosted.
2. When you say after "the upgrade", what upgrade are we talking about v18 to v20 or V20 U6 to U7?
Given the small amount of information you have provided, I'm going to guess the following:
1. Sour sever is NOT hosted on the 3CX hosting platform, rather, it is hosted somewhere else like Amazon.
2. Further, given that you are unable to login or reset a password after the upgrade, I will assume that you are talking about upgrading from v18 to v20.
I don't know who performed the upgrade, but there are very specific MANDITORY steps you must perform BEFORE you upgrade. from V18 to V20. You should have assigned the System Owner role to at least one user, AND ensured that every user extension has a unique email.
If you did not perform these steps prior to the upgrade, then you would not be able to login, nor could you reset your password.
There are a couple of 3CX blog posts that clearly outline the prerequisites. Here is one:
https://www.3cx.com/docs/upgrading-pbx/