Hello,
my RB450G / Routeros 6.20 is working fine (for years). However, last week I've suddenly lost access to its administration, despite the router seems to work normally otherwise.
Winbox: Logging in (several minutes), then "Router has been disconnected"
Webfig: I enter the credentials, and after a minuter or so of "Loading" there is an "Internal server error".
Telnet: I enter the credentials, and after minue or so "Login failed" (I have not changed the credentials).
The problem arised, after I attempted to connect to the routerboard via Winbox v. 3.18 (previously I have used Winbox v. 2.2.18 with no problem.)
Is there any way to regain the admin access without rebooting the router (we a running a live radio station broadcast stream through it, so any interruption is undesirable)?
Thank you.

Such an old version with potentially exposed admin interface makes me think of this:
https://blog.mikrotik.com/security/winb ... ility.html

Make sure to keep your devices upgraded.

As for this one, try Netinstall. It you have no access, there is not much else you can do.

I think you should try and reboot and if you still can not access your device then as normis said do a netinstall...