IMac dead. Just a grey screen. No access to serial number, net, email. What to do?

iMac, 6 years old. Upgraded 2 years ago, more storage capacity (40.000 photos), faster processor. The power has diminished little by little. Now no access to files, net, email. Just avgrey screen. What do you suggest? Ner Mac Mini box? Luckily I have an external memory, no loss of files, but obviously too much information for an old computer. Please help!