Standard currency problems [RESOLVED]

Can someone help me with this issue?

Out of the blue, the currency format of my forms changed to USD. (used to be Euro). Also the decimals changed. A product price that used to be €356.95 changed into $35695,00.

When I try to change the standard currency to Euro again and save the settings, it displays so until I refresh the page.

Is this a known problem? Does anyone have a fix?

Please see the following:

Thans for the help. I added the text to the functions.php file of my theme and now the currency is euro again.