Is prayer with tayammum valid when water is cut off and insufficient for ghusl, and is it obligatory to perform ghusl and repeat the prayer when water becomes available again?

If you performed Tayammum without sufficiently searching for water, your prayer is invalid and you must repeat it. If you searched for water and did not find it, and you have enough water for some of your limbs, then you must use it and perform Tayammum for the rest. This is the strongest and most cautious opinion. Therefore, your reliance solely on Tayammum in this situation is not sufficient, and you must repeat the prayer, in addition to performing Ghusl from Janabah when water becomes available.