No Bluetooth connection on iMac keyboard and mouse

I have not used my iMac computer for a couple of weeks. Having switched it on tonight I thought it might be prudent to check how much power was left in the Bluetooth keyboard and mouse.

in error I managed to switch off the Bluetooth connections and now I have no keyboard or mouse, I feel such an idiot, have you any helpful suggestions please. TIA

[Re-Titled by Moderator]

Original Title: No Bluetooth connexion