Today, for no obvious reason, my IDE (2.2.1) is saying
avrdude: ser_open(): can't open device "\.\COM7": Access is denied.
If I move to a different usb slot, the IDE still shows com7
I've rebooted, no change
Did that include turning everything off, then back on again?
a7
yes, power down
Could port 7 be in use in another app? "Acces denied" makes me think of it.
Double IDE running and You try from the wrong one?
Closed and opened the IDE?
I've faced this as well but have forgotten why it happend.... It's rare...
You can also try updating the drivers if all else fails.
uninstalled com7, re-added, all good 
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