What do Hrothgar and his council do to try to save his guest-hall?

I think that the answer you are probably looking for is that they performed sacrifices to the gods to try to save the guest hall.

You can find this in Chapter 2 of the poem, right at the end.  Grendel has been killing Hrothgar's thanes and they have not been able to figure out any way to stop him.  So what they did was to sacrifice at "heathen shrines" to ask the "slayer of souls" to help them.

Since the poem was written down in Christian times, the poet criticizes this practice, saying that it was hellish and that they would have done much better if they had been Christian and known the true God.