Sir,
My garage which is I ft below the road level in rainy season, when the underground drainage is blocked the water get stacked until the drain is clear making a mess to my vehicle. The height of the garage is 6 ft and my car height is 5 ft. Please give suggestion how to solve out the problem

Not enough detail to determine whether you are talking about surface water flowing down the driveway and into the garage, or a rising water table after heavy rains causing the problem. Or combination of both.
If it is water coming down the driveway, a slotted drain* placed below and across the driveway at the road edge may help with the problem, if there is a point downgrade you can drain the slotted drain to.
* http://www.conteches.com/DesktopModules/Bring2mind/DMX/Download.aspx?Command=Core_Download&EntryId=1948&PortalId=0&TabId=144

If it is a rising water table you are talking about, underdrains (stone with a perforated pipe at bottom of trench) or french drains (stone only or with pipe) can help- IF a lower point is available, to drain to.

Sounds like you are considering raising the garage floor. May be possible but you would have to look at possible problems that could create and it would definitely be the most expensive option with garage floor overlay, and driveway modifications necessary. If it is surface water causing the problem, raising the floor wouldn't help.

when the underground drainage is blocked Are youj saying that your entire drainage system isx overloaded during a heavy downpour? Is your sewer connected to your storm drain?
the garage is 6 ft and my car height is 5 ft. What does thatg mean? Before we can offer a solution you have to give us more information. Back to you, Tom

"when the underground drainage is blocked " what is the cause of the blockage?

Are you the only one that's so affected during the rainy season in your neighborhood? Is this by chance Africa or India? How old is your neighborhood you live in? How long is this rainy season? Is your garage big enough to accommodate ramps for your vehicle during this rainy season?

Exactly how many inches of water are we talking about and how long does it last, and is your entire street and block flooded as well? I highly suspect we are not dealing with a western idea of your sewer system, and may not fully appreciate your dilemma.

INFORMATION! DETAILS! That's what we need before we can answer with a solution. Back to you, Tom