Random error establishing connection to database on 3 websites

Hello On 3 websites I have running on a WHM server using cPanel over the last few weeks I have an "error establishing connection to database". This error has appeared out of the blue with nothing being done to any of the websites as they are all brochure sites.. They instantly fixed themselves when I went into the control panel and repaired database. Would anyone know what the possible causes may be for this as this has never happened before. Also are there any logs I can access which will give me more indication of the original problem and where do I find these? Many thanks, William