Can viruses be reintroduced to humans from ice sheets?
Yes, pathogens have been shown to survive freezing temperatures under ice and still remain potentially dangerous after the melting of ice.
This has not only happened for relatively simple molecules such as the ones that viruses are made out of, but it has also even happened with larger pathogens such as bacteria (anthrax outbreak in 2016).
Unlike high temperatures which can cause molecules to dissociate due to excess kinetic energy, freezing temperatures usually preserve molecules and even cells such as with the aforementioned anthrax outbreak. This means that viruses can survive cryogenesis.
In 2015, researchers in Tibet discovered 28 'new' species of viruses in (melting glaciers).
These sort of discoveries lead many to believe that with global warming, previously unknown viruses could appear once again.
As for SARS-CoV-2, it seems that melted ice sheets are unlikely to be the source of such a virus considering (among other reasons) that the virus was first noticed in Wuhan China, a place lacking permafrost.