For an answer specific to your machine we need Make/Model/Year/Engine Model/Transmission Model/Deck Size, and hours on meter if you have one. A short description of the events preceding the beginning of the problem is also helpful.
Here are some general thoughts on Hydro Transmissions
The NOT SERVICEABLE UN-TRUTH.
The Huskvarna YTH150 uses Hydrogear transmission 319-0650, drive model 150071. One place to find parts and diagrams for most Huskvarnas is here: http://www.m-and-d.com/Hydro-Gear_parts.html
MTDs use Hydrogear trannys also. Go to the site and enter 618-0319 in the search box. It will bring up a selection of MTD transmissions.
Sears Craftsmen mowers also have trannys from Hydro-Gear.
The best lawn tractor Hydrostatic Transmission Fluid isactually 10W-50 synthetic motor oil (like Mobil 1). Do not use hydraulic fluid or transmission fluid.
Hydro static transmissions on most lawn tractors are very similar. Most of them are made by one or two manufacturers. Tuff Torq makes a lot of them, but not the Huskvarnas. They do make all the John Deere lawn tractor hydro drives. The lawn tractor manufacturers (like JD) often say they are sealed and not serviceable. That is flat out, an untruth. All of the JD hydros, and most all others, are serviceable. A complete line of rebuild/replacement parts can be found at Tuff Torq. Info and instructions can be found there. They have a lot of good general info for anyone rebuilding their tranny.
Additionally, I have found that the mower manufacturers fill them (or spec them) with motor oil, 10W-30W. In the service manual, and at the hydro manufacturer, you are told if your tractor is used in extreme or heavy duty situations it's best to drain the oil and replace it with 10W-50 synthetic motor oil (like Mobil 1™). Truth is the tranny should have this oil from the get go. It will give you more power and extend the life of the tranny. The mower manufacturers are all just being cheap. They figure why not, you're the one who has to pay for the early failure. Oh yeah, one other minor detail, the tranny manufacturer also recommends regular oil changes, like every 100 hours.
Virtually all of these trannys have a black drain plug on the top, if they do not have the add on reservoir. You have to remove the tranny from the mower (not a difficult things to do, it takes about 30 min). Pull that black plug (I used a 1" putty knife to pry mine out) and turn the tranny upside down for about 30 min to completely drain. Most hold about 2 quarts. Flip it back, right side up, and refill. Install plug and reinstall tranny. Note: under the plug there is often a magnet which collects metal shavings. Lift this magnet out while you have the plug out; clean it and replace it. It may fall out while the tranny is upside down. If anything else falls out, you have a broken tranny and will have to take it apart.
When you refill the fluid just keep filling till it comes up within a quarter inch of the plug. You may have to go through a bleed process after reinstalling the tranny. The tuff torq site explains the bleed process.