Cannot login-Reservation of Controller failed
Check Control Builder Session Logs
Right Click Controllers and "Show Remote Systems" - Check the loaded applications
Check the controller logs
Can the OPC Servers save the cold retain values ?
Can you go online without download ?
Have you physically checked the LEDs on the controller ?
Have you got another engineering station.
If there was a problem during download and the download procedure crashed somewhere, then unfortunately you usually do have to restart the PLC to resolve the problem. But the controller should continue to run the previous version of the program meantime, so don't panic, take your time and collect and analyze as much information as you can.