localhost / 127.0.0.1 suddenly(!) not working anymore (in general, not only for XAMPP)

Question

I'm encountering a sudden problem with my localhost / 127.0.0.1 access.

Situation is as follows: I'm using localhost / 127.0.0.1 for my developement enviroment server (XAMPP), so i can access the in-progress version of my project via browser. I also use it for for some other programs, not only for XAMPP. Last night, when i stopped coding everything was still working fine, like everey day. But since today localhost is not accessible anymore - not via browser, not for other programs which used to use it.

• The hosts-file of Windows was NOT changed since when everything was functioning well
• The Problem occours not only when i try to access via browser / XAMPP. Seems to be a general localhost adressing problem
• Only action between working and not working state was turning off my computer last night and turning it on again today
• I've tried to re-install XAMPP and other relevant programs, still not working
• Same problem applies, when i try to access 127.0.0.1

I absolutely have no clue, what could'd be responsible for this. Has anybody a suggestion? Would be very happy about that!

PS: Sorry for my possibly bad English.

PPS: StackOverflow is such a cool website, it helped me like NO other in the last years with getting better in coding. Thank you all!