Pushrods don't just "slip" off the lifter.

It sits in a cup on the lifter and in the rocker arm. Valve spring pressure
holds them all together....until . . . .

So, what happened just before it "slipped"? A downshift at too high of a
ground speed? a "wanna see how high it will rev" situation? Is the
valvespring broken? Valve swallowed?

In the first two situations, the pushrod is probably bent and won't stay in
place (replace it), in the second two, repairs are needed before you put it
back in.

The final scenario is one where you were working on the engine and it
slipped off while tightening the rocker shaft . . .loosen it up and slip it
back in and retorque.

Like Budd said, the first step would be determining why it jumped off.
Remove the pushrod from the engine and inspect it for damage (make sure it
is not broken or bent).  Then inspect the rocker arm and make sure it is not
cracked or damaged.  You might also want to make sure that the valve is
moving properly and is not sticking.  If everything seems to be ok, loosen
up the assembly, put it back in, tighten it up and see what happens (run it
for a while without the cover(not drive it)).