Felt like something popped in upper thigh so now I can't lift leg or run but can walk. Whats that?

Flexor/groin pull. You may have strained your hip flexor muscles (psoas/rectus femorus) required for 'lifing your leg or running' a 'groin pull' is also caused over-stretching of the adductor muscles (which include the adductor magnus, longus, brevis..) treatment is r.I.C.E : rest, ice, crutches, elevate then slow stetching. See also http://www.Sports-injury-info.Com/hip-flexor-injury.Html.