|Does this happen to all PC's and tills at the same time, or at different times?|
If all at the same time, the most likely suspect is your router. The next time this happens, power cycle your router (ie: unplug the power adapter for 10 seconds, then plug it back in) and check, is everything working properly now? If it is, then it's likely your router.
If the problem persists, update the firmware in it. If the problem still persists, you may need to replace the router.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.