What does this exactly mean?
As @Tigran Balyuan indicated, 'DISCONNECTED' is a vague term to inform you the NAS couldn't communicate with one of the drives. It could mean the drive failed, the SATA (or power) adapters are not seated properly or the controller that's reading the drive is bad. No way to tell unless you physically inspect the device, but in most cases (in my experience with this message), the problem is due to the NAS and not the individual HDs.
Is it possible as the HD will work again?
Usually, if the HD is bad, the system will give you a more specific error; like the HD failed or has bad sectors. Restarting your system will give you a quick indication of whether or not the RAID integrity is in-tact. If it is and you get a more specific error about a particular HD, you can replace the HD and the system should automatically rebuild the RAID on next reboot.
Would this resolve the RAID5?
There's no indication that restarting or replacing a HD will resolve a problem. RAID5 just indicates your striping and mirroring configuration. RAID5 is generally resilient enough to stay functional up to 1 failed disk. If you need anything more fault tolerant, I'd suggest going to RAID1 or RAID1+0; note; going to RAID1 or RAID1+0 is generally more expensive, because you're sacrificing more disks for tolerance.
Do you suggest me to do this? Or should I just turn off the NAS until the HD is replaced?
Rebooting a NAS is basic a troubleshooting step. However, someone really needs to physically inspect the NAS. There might be light indicators telling you which drive is bad; or when the system is shut down, someone can inspect the connectors to verify solid contact. If the system sees a problem, you'll be in the same spot you are now. If you replace the bad HD, the system will automatically rebuild the RAID from parity bits.