Is worker’s compensation the only claim I'm entitled to if I fell into a manhole while at work?

As far as your employer, yes, you are likely limited to workers' compensation. The question is whether you would have a third-party claim. Was this manhole something that was at your place of employment? Was anyone else responsible for its condition? Is there any argument it was defective? Regardless, there is not any way you will do well without counsel. Your fees on the workers' compensation aspect are set by statute and you are highly encouraged to have representation from the outset since each statement you make, each doctor's visit, ..., will impact your ultimate claim. Pull together your records as best you can. If it is possible to photograph the site of the manhole from various directions and angles, you should do so. If possible, include daylight pictures, as well as pictures that would show similar lighting and weather conditions to what they were at the time of the accident. Set a consultation. In analyzing your workers' compensation claim, other possible claims will be considered. One will, however, offset against the other. So, you would gain something if you are not limited by the structure of workers' compensation, but you will not be collecting from two different sources without an adjustment.

You may have a third-party claim against whoever owns or maintains the manhole that you fell into. You should immediately investigate the possibility of a third-party claim, because the statute of limitations (time limit for filing a claim) for the third-party claim could be substantially shorter than for your workers' compensation claim (particularly if the manhole is owned or maintained by a governmental entity).