Error 500 after creating a sub assemblee which disappears from the assemblles list

When : creating a municipales assemblee in a regional assemblee It created the assemblees. I create a municipales type, then try to set the new one to it And I got this "500 Internal Server Error If you are the administrator of this website, then please read this web application's log file and/or the web server's log file to find out what went wrong." And the municipales disappeared from the list of assemblees. I tried to set a new one but at this time the old one and the new one appears again but when going to the site I got the same error message. No way to get the landing page, so to login But the old logged in is still in process working and I can have the administrative board. How could I get back from this trapped? Thanks