What is the ruling on a colleague of mine at work suddenly cutting me off and refraining from returning my greeting, after our relationship was good?

It is not permissible for your colleague to shun you without a legitimate reason. If you can reconcile with her, then do so. Otherwise, the sin is upon her, and you have cleared yourself of blame. The Prophet, peace and blessings be upon him, said: "It is not lawful for a believer to shun a believer for more than three days. If three days pass, he should meet him and greet him. If he returns the greeting, they both share in the reward. If he does not return it, he has incurred the sin."