console into the 5510 and take note of telnet access, it is very possible something changed on the hosts that used to access the asa like the IP address.
e.i, say you are accessing asa from inside hots ip 192.168.1.20
asa(config)#telnet 192.168.1.20 255.255.255.255 inside
or if you want to avoid being so granular in which hosts on a given interface can telnet to asa you can simply allow any.
e.i
asa(config)#telnet 0.0.0.0 0.0.0.0
HTH
Rgds
Jorge
Jorge Rodriguez