If you have system restore turned on, you could just go to the latest restore point from before you did this and restore it. To run system restore, assuming it was activated, Start, search type "system restore" hit enter, and choose an appropriate restore point and hit OK and let it do its thing. Otherwise:
If this is regarding the native windows fax service, you could try the following:
Start, search, type "windows features" and hit enter. Then in the list that appears look for "print and document services" and expand that folder and put a check mark in "Window fax and scan." Hit Ok and hopefully it will be back. Might require a reboot.
If that doesn't do it, you could try repairing system files by running the system file checker. To do that open an administrative command prompt, type "sfc /scannow" with the space and w/o the quotes and hit enter and let it complete. And then if it fixes anything, try repeating the first procedure.
If that doesn't do it, a repair install
may be necessary. It takes a while to complete but it leaves your files, apps, data and most settings in place.
If it is an external fax machine you are talking about, just unplug it and plug it back in and windows should automatically find the driver
and install it.