The symptoms you describe sound like all the vty ports are in use. This may happen if you have configured the vty ports with no exec-timeout (or with exec-timeout 0 0 which is the same thing). If you can get to the console port the show user command (or the who command) should indicate which vty are in use - and I suspect that they all are.
If you can get to the console port and enable mode you could clear some of the sessions which would make telnet work again.
If you can not get to the console port you might wait and try later and see if it works after someone logs off. The other alternative is to reboot the router.
While I do sometimes configure the console with exec-timeout 0 0, I generally advocate not doig it on vty ports - mostly for the reason that you are experiencing. I sometimes put a very long timeout on the vty, but setting it to 0 makes me nervous.
This document gives several answers on frequently asked questions for PFRv3 channel state behavior.
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