First, you'll have to locate the piping that supplies water to the tub. There should be two lines (hot & cold) - you'll have to trace them back to where those service lines branch off from a main service line. Look for a valve used to control flow to that service branch. If they closed-off a valve on this line, opening it is the easiest likely solution to the problem.
If whomever installed this tub and piping only supplied hot water, you're looking at a plumbing job - but more information is needed for that.
Perhaps posting some pictures would help us to visualize your situation better.
If I can be of further assistance, or if you have questions about any of my response(s), please let me know before rating me.
If this solves the issue, please rate it accordingly. My goal is to earn a "Very Helpful" rating from everyone I help.
Answers & Comments
First, you'll have to locate the piping that supplies water to the tub. There should be two lines (hot & cold) - you'll have to trace them back to where those service lines branch off from a main service line. Look for a valve used to control flow to that service branch. If they closed-off a valve on this line, opening it is the easiest likely solution to the problem.
If whomever installed this tub and piping only supplied hot water, you're looking at a plumbing job - but more information is needed for that.
Perhaps posting some pictures would help us to visualize your situation better.
If I can be of further assistance, or if you have questions about any of my response(s), please let me know before rating me.
If this solves the issue, please rate it accordingly. My goal is to earn a "Very Helpful" rating from everyone I help.
Thanks for using Fixya.com,
Jon