Thank you for your question.
There is not a real accurate way to check suction power on a vacuum cleaner as a home owner.
In the industry the vacuum repair shop may have a water lift measurement tool, but that is not the whole picture of vacuum power either.
Since we do not have a Model number of the vacuum,I can only give you general things to look at.
Since you have said that your vacuum has suddenly lost power, there are a couple of things we can check to see what the problem could be. Most problems are related to air flow blockage.
All bags and filters need to be checked/cleaned or replaced.
All hoses and wands and floor tools need to be checked for air flow blockage.
First we need to consider whether it is a upright or a cannister style vacuum cleaner, because we approach the problem in a different manor.
If it is a cannister type, the first thing I would do is remove the hose from the cannister body and turn the motor unit on and check for how much suction you have right at the hose connection. Some models will only work with the hose assembly in place. What we are looking for is to see if we have good suction at the motor unit. One way to check this is to put the palm of your hand right over the inlet hole and see if the motor speed increases. This would indicate we have good suction at the motor unit. If you do not it would indicate that the motor unit is failing,or we have so internal air blockage.
Some hose controls and power cannister units have a variable speed control either on the cannister body or on the handle control so you can dial down the suction for vacuuming drapes.
For upright vacuums you need to check many of the same things, but most of the problems are air flow in the air flow path.
Check the roller brush belt to make sure it is rotating freely and when operating the vacuum it is rotating when in contact with the carpet.
Remove the dirt box cover or dirt container and start the vacuum motor, this will free the motor of air paths and the motor should run at full speed.
Then starting at the roller brush on the floor, turn the vacuum up side down so you can either remove or examine the air path in the roller brush area and then working your way up the air flow into the dirt compartment. Depending on the model type a short hose that comes from the floor brush feeds the dirt into the dirt container. This is a common area for vacuumed items and dust to travel and possible get lodged and build and cause a blockage.
If you have checked all this and have not discovered the problem, I would recommend taking it in for service. I have included a link for the service drop off locations.
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services