i've never experienced that. Usually it just gets really really hard if you don't cover it. You could try adding water to it - a few drops at a time to get it back to icing consistency.
For the future, I usually store my royal icing in tupperware or the plastic take out soup containers.
HTH!
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Have you tried adding some water to your RI and mixing well?
I would try to add a small amount of water, it might help, but it sounds as if you got some grease in when mixing, then there is not a lot that you can do to save it.
when making RI make sure you use a glass bowl and that your tools are free of grease. I rinse all my stuff in vinegar water to make sure its grease free.
HTH
Every time you leave RI for more than 20 minutes you need to beat it again to get it the right consistency (can do this with a spoon, not necessary to use a mixer). Always place cling wrap directly onto icing surface and into an airtight container if you need to store it. Add in a drop of strained lemon juice or egg white at a time while beating to get it smooth and 'just right' again if its too thick and gummy.
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