It would be help (in fact, essential) that you produce the code you used for at least NonlinearModelFit.

Generally when fitting sine waves to data with NonlinearModelFit, there needs to be a reasonable guess at the parameter representing the frequency of the sine wave. The default starting value is 1 for all parameters if you don't give starting values.

If your model is a simple sine wave and the time values are in days, then using 2 Pi/365 will likely be a good starting value for the frequency parameter.