If the problem is fat, you'll have to lose it and that means calorie control. You must control your energy (calorie) deficit to burn fat. There is no other way. Exercise is not a panacea because you can always eat more calories than you can burn.
If the problem is loose skin, there's nothing you can do but wait and hope there's sufficient elasticity to allow it to tighten up again...a process which may take several weeks. It will depend on how much fat you lost from your abs, your age, how much fat remains to be lost, etc.
Check out these ab facts.
• Your abs do not need exercise. They are working for you most of your waking hours helping contain your internal organs and preventing your spine from collapsing like a stack of poker chips. Of course, you can still work them harder or more if you want to.
• The reason abs are not visible or prominent is subcutaneous (under the skin) fat. If you have no abdominal (belly) fat, you will see your abs when the muscle is contracted.
• The only nonsurgical way to lose ab fat is through calorie control and that means diet.
• There is no nonsurgical way to remove fat from only the abdominal area. The body and genetics determine where fat leaves when a person reduces body fat through calorie control.
• There are no supplements, pills, topical applications, spa treatments, etc. which will remove fat from the abs.
• Sit-ups, ab crunches, or abdominal exercises will not remove fat from the abs any more or better or faster than jogging, lifting weights, or running on a treadmill.
• When burning fat, the location of the muscles being exercised has nothing to do with where fat is lost. Spot fat reduction is a myth.
• Ab fat is usually the last fat to go when a person loses fat. That means for prominent fat free abdominal muscles, body fat will have to be reduce to about 10% or less. It also means there is no fast way for a person with ab fat to get prominent abs without surgery.
• If a navel is concave (an "innie") or dimpled inward, it is surrounded by ab fat. The deeper the navel depression, the more ab fat there is around it. The more ab fat, the longer it will be before the abs can be visible or prominent.
• The only nonsurgical way to lose ab fat is to use calorie control to lose fat and then wait for the fat to leave the abs.
• For most people, there is no nonsurgical way to lose ab fat fast or easily.
• Abs, like any other skeletal muscle, can only grow in size and strength through increased resistance training/work.
• Not all people have a "six pack". Some people have 4, 5, 6, 7, or even 8 heads to their rectus abdominus muscle.
• Most people never get six pack abs because it's just too hard and not necessarily good for their health.
• Just about every website you see if you Google "six pack abs" will be bogus, misleading, or contain a mix of good and bad advice and/or information.
• A cut six pack abdomen is not necessarily a sign of good health. In many cases it's a sign of poor health.
The truth is, unless you're very young (teens) and have the genes of a near ectomorph, you will probably find that a ripped abdomen is not for you. It will require too much sacrifice for too little gain. No one needs a prominent six pack so you will find almost no information about six pack abs on any of the scientific or government web sites. They don't care about your vanity. They care about your health. That's why most of the information about six pack abs on the internet is bad information or scammers trying to sell you supplements.