Hi Sylvia,

Most often means that there was an invalid or misspelled backslash () character in the code, which the PHP parser did not expect.

Probably somebody edited php file from Joomla core, Template or Extension (Module, Plugin). Or you changed PHP version. Nothing happens by itself, there is always a reason.

1. Is your Joomla admin area still works?
2. Please reinstall Joomla core files
3. Change template to different one, and compare front-end.

And as always - use Joomla Debug mode

Remember that forum.joomla.org have many topics about that already