Hi,
 
When I want to use SENDS to send multiple channels to a REVERB channel, do SEND to an AUX channel or STEREO BUS?
 
I had an AUX channel but now out of the blue, NONE of the sends want to work...

I use buses because Sonar didn't have Aux tracks a few years back when I started with X1.  I don't know that one option is better than the other for Sends.

Reverb send to a stereo Buss works fine.

Either will work, FWIW.   But for Aux's in particular, make sure the Input Echo button is enabled. 
 

I'm new to Aux tracks but I already have a definite policy on the use of AUX v BUS, as follows:

• If I will be needing to adjust settings later during the mixing stage, then I use a BUS.
• If I intend leave it alone once it is set up, then I use an AUX.

But I have not really explored the possibilities of Aux tracks yet, so this may change.

Thanks for the input (pun intended)

Interesting...
Coming from live consoles and being a long-time user of another DAW, Auxes and Busses are conceived of differently. 
Typically, you would use an AUX for Effects, as then you can send many channels to a particular effect (i.e. Room or Hall or Plate reverb) or an Effect Chain (i.e. delay, verb, compression, shelving EQ, etc.) and determine the AMOUNT you desire to send per channel. For example, it would be standard to have the reverb set at 100% on the Aux channel, and then send differing amounts to taste from each source channel (i.e. maybe -20dB on a Lead Vocal, and -15dB on backup vocals - to highlight the lead vocal). 
 
Meanwhile, think of Busses for Grouping Channel Outputs and for mixing. So, any groups of channels that you want to send through the same EQ or Dynamics chain - and control for volume as a group (i.e. Drums, Guitars, keyboards, orchestra, backing vocals, etc.) - you assign their outputs to Subgroups. 
 
Then software that emulates hardware VCAs becomes another sweet option, for controlling groups of faders (but no audio passes through). It's worth thinking of WHY such a category exists and what advantages it has compared to another category. 

Both will do the job so I think it is more of a question of which "workflow" / organisation you prefer

Do you want the "fx busses" to sit with the other busses in the track view so that they are separated from your tracks, in which case use busses
Or
Do you want to integrate your aux tracks within your track - eg you could put your vocal reverb etc. in an aux track and organise it within your vocal track folder

I personally prefer the bus approach - but as someone else said I can see advantages of putting your aux tracks with your tracks if you are going to set them and forget them

It is great the SONAR gives you the options to suit your workflow