It's common for slimes to arrive stiff due to cold weather. Most slimes can thaw out within a couple hours if you leave them in a warm place or by warming it up with your hands. In more extreme cases of stiff slime you can try the following solutions:
White Glue Base: Mix small amounts of lotion in slime and knead until your slime has reached your desired consistency.
Clear Glue Base: Keep slime in an airtight container in a warm place for a few days to allow the slime to soften up. You can also try adding small amounts of glycerin which will soften the slime.