iMac will not connect to laptop

Imac and MacBook Air on same network. At times both show up in each other's finder as Shared. I am unable to connect from/to either. Times out or get an error saying "There was a problem connecting to the server, " xxx" The server may not exist or it is unavailable at this time. Check the server name or IP address, check your network connection, and then try again.

HELP.