Error message «wait until importation is done»

My iphoto has been showing an error message ever since my last importation from my iPhone to my iPhoto library telling me I have to wait for the importation to be done before I can : import new photos / turn off my iPhoto / Turn off my computer at all. Basically, I cannot do anything. But there is nothing plugged to my computer, no importation running, nothing. What should I do ?

Thanks a bunch !