On my Topcon rl-sv2s the instrument will spin but there is no red beam how can I fix this

The RL-HB was only a self leveling laser and the newest Topcon equivilent would be the RL-H5. A higher spec and with a grade function.

There are 2 small screws holding the rotating head motor in place. Likely you just need to loosen these screws, and adjust the motor position and retighten.

http://www.benchmarksupply.com/SUPPORT/RLH4C_RevE0124.pdf

for as long as the link is valid here is a copy.

If you have the standard RL-H3C then there are 3 buttons. Starting from the left there is the "height alert" then "manual" then "Power".

Height alert is a function that after the laser levels and spins monitors the position of the laser. If the laser moves more then a tiny amount then the height alert function is tripped, and the laser flashes it's lights and stops spinning (or spins slowly) to let you know the base line position of the laser has changed. Pressing the height alert button twice shuts this mode off.
Manual mode means after the laser is leveled, it completely ignores the level vials. You can turn the laser any way you want to and it will always spin and not cause any errors.
Power is self explanatory.
If it's a RL-H3CS then it has two arrow buttons, that when in manual mode, allot one axis of the laser to be manually adjusted up and down to do a manual slope.

There are detailed instructions in your users manual on how to check and adjust your Topcon laser. You'll need a tripod, 2 vertical surfaces about 50 meters apart and your laser receiver. It's done by marking positions of the beams, on the different surfaces and then adjusting your laser position to the center of the different positions.
The procedure is different for different models, but i have attached some pictures from the manual for the RL-VH4 series.

Take this to a survey/laser repair shop, preferably one that does not charge for estimates.. The laser diodes on the RL-H3C is not serviceable. It requires the lens barrel assembly (or telescope tube unit, i don't remember what they call it in these). It's unreasonably expensive, and will likely be a write off. It may however be the main circuit board, but likely not. Be prepared to replace this unit.

How old are the batteries? The company appears to be proprietary and does not release much information.

I have one of this model too and have the same problem. It spin and spin but won't level. After a couple of minutes it stops and the power led fast blink.

Hopefully it's the simple answer. The bubble vial may be out. start where the level vial says is level, and move outwards in slow circles to try and find a place the laser thinks is level. if you find a spot, use the 3 adjustment screws on the bottom of the bubble vial to make that your new center.
Otherwise, it's a not good news. A couple things could be gone wrong. The oil bath compensator could have leaked. If you open the laser, and it's oily inside, throw it away.
It could be the peep site could need adjusting, or the laser diode could be blown. If the laser diode is dead, same solution as the compensator leaking. There are no parts available for these anymore, chuck it out.
The peep site isn't a write off, per se. The laser operates by shooting the beam through a oil bath, which compensates for the instrument not being perfectly level, and then through a splitting lens. part of the beam travels up, and through the rotating head. And the rest goes through a peep site, to a light reflecting area of the main circuit board. if the board or peep site is no longer aligned, the laser will never think it's level. The bad news about that, it is a very tricky and time consuming repair, that few technicians know how to do, or if they do know how are most aren't comfortable doing it.
You can try taking the cover off, leveling the instrument, powering it up and trying to move the circuit board until the laser thinks it's level, but i wouldn't waste much time on it.

Sound like the laser diode is failing. It's power is likely just strong enough to get the level position right, then dims as it runs. The unit, no longer seeing the laser, decides it's out of level and stops. Or the oil in the compensator is leaking, and the same process is happening. Unfortunately the replacement part is not economical to replace.