I followed your advice and it worked,the invoice is back to the way it is, thank you for your help.

Hi Melekeddine,

Usually this issue is happening becuase your web.base.url or report.url is invalid. Activate your developer mode and go to settings > Technical > Parameters > System parameters. Make sure that you have either a web base.url pointing to your Odoo instance (127.0.0.1) or that you have a record report.url which has the value 127.0.0.1.
If you work with the web.base.url you might want to create a new record web.base.url.freeze with the value set to True. If you use the web.base.url and have no web.base.url.freeze configured the web.base.url will change when you acces your Odoo from another url, for example mywebsite.com. When you use the report.url it will only be used for generating the reports. An example:

Regards,
Yenthe