It depends on how hot you got it, it will probably still work. I just sprinkle it on top, that's what I do with all my dry yeasts and they end up working just fine. To rehydrate something properly it takes too much time and effort and increases my risk of infection.
1. Suspend yeast rehydration nutrient in 20 times its weight of clean, chlorine free 43°C(110°F) water. If the water
temperature is not high enough, the yeast rehydration nutrient may not go entirely
into solution. We recommend Lalvin GoFerm and Laffort Dynastart as appropriate
yeast rehydration nutrients.
IMPORTANT:
If not using a yeast rehydration nutrient, water temperature should begin at
40°C(104°F) to avoid harming the yeast.
2. Once the temperature of the yeast rehydration nutrient solution has dropped to
40°C(104°F), add active dried yeast. Stir gently to break
up any clumps. Let suspension stand for 15-30 minutes, then stir gently again.
Allowing rehydrated yeast to stand for more than 30 minutes may start a decline in
the live population.
Note: Foaming is not an indicator of yeast viability.
3. Slowly (over a period of 5 minutes) combine an equal amount of the must/juice
to be fermented with the yeast suspension. This will help the yeast adjust to the
cool temperature of the must/juice and will help avoid cold shock caused by a
rapid temperature drop exceeding 10°C(18°F). This atemperation step may need
repeating in very low temperature must. For every 10°C(18°F) temperature
difference between the must/juice and the yeast slurry, an atemperation step must be performed. For example, for a
must/juice temperature of 20°C(68°F) and yeast slurry temperature of 40°C(104°F), two atemperation steps are
required.
4. Add the yeast slurry to the bottom of the fermentation vessel just as you begin filling the vessel with must/juice.
This is especially important for large tanks with long filling times or when inoculating with strains that are sensitive to
the competitive factor. This will allow the yeast a head start over indigenous organisms.
paraphrased from
http://www.scottlaboratories.com/info-center/documents/vC-YeastRehydrationProtocol_000.pdf
