You need to give a bit more information, I am afraid.
the critical path is not something that you make, it is the set of tasks that have no float on them, or to put it another way, if any of them slip by any time, the finish date will change.
Generally there will be always be a critical path. the main reasons for not seeing one are constrained dates (these will skew the programme), or set start and finish dates that are further apart than the duration of the programmed items. In this case, generally you would add a float bar- this links from any task that does not have a successor (something happening after it) and links to the finish date. The duration of this can be set so that it takes up all the slack in the programme, and this will then allow the critical path to be shown.

If you need more help, then an example would be ideal, or a bit more explanation as to the exact problem.

hope this helps.