On what grounds would my job have to let me go?

I am currently in limbo at my job. I have been at my company for 5 years. I have received outstanding grades on my all of my performance reviews from my manager. About 1 month ago I accepted a position in a different department but learned within the first few days the job was not what I expected and was not a fit for me. I consulted with Human Resources to let them know about the situation and that I wanted to return. I then asked my former manager if I could return to my since the position had not been filled and I was still helping him out anyway. He said yes. I informed my new managers and began working with Human Resources who instructed me to not return to my new position since I told them I was no longer interested and to wait to hear from them before returning to my old. I kept in touch with my former manager to let him know what was going on. A few more days after that my former manager stopped returning my calls and for 2 weeks now HR has told me they would have an update for me but never give me that update. I never formally resigned (nothing written) from the new job and I gather my former manager must have changed his mind. I realize this is not looking good, I'm still on the payroll but running out of leave. Are there any HR employees out there or anyone familiar with their general practices? Is HR just looking for ways to let me go? What could be taking so long to make a decision? What should I do next? I'm at a lost.