If your Evo was rooted, you'll need to root it again. (and not install OTA updates)
If your Evo wasn't rooted, and you were using the free tethering over 4G that Sprint was offering, that was due to stop being free in July.
If your Evo wasn't rooted, and you're paying Sprint the $29.99 monthly fee for tethering/wi-fi hotspot service, you need to contact Sprint.