Bluetooth Connect Reconnect Problem / Solution Workaround for Yosemity: (Summary: Try 'Turn Wifi Off' to wake up Bluetooth!!!)
If you can not connect/reconnect your bluetooth devices, try "Turn Wifi Off". It seems to 'wake up' the bluetooth and then my devices reconnect.
In my case, after sleep/wake of my MacBook, my bluetooth keyboard would NOT reconnect. When I 'Turn Wifi Off" my bluetooth starts working again immediately and my bluetooth keyboard immediately reconnects. I read on some non-Apple.com forum that sometimes 'Turn Wifi Off' on yosemite makes bluetooth start working again. Other times, 'Turn Off Wifi' followed by 'Turn On Wifi' makes bluetooth start working again.
In my case, whenever I can not get my bluetooth devices to connect/reconnect to my MacBook, I simply "Turn Wifi Off" and it immediately reconnects.
Good luck.
P.S. I'm guessing that with the 'Handoff a.k.a. Continuity' enhancements to Yosemeti, which use Bluetooth to dynamically enable iOS devices to coordinate workflows with MacBooks, that Handoff's dynamic explolitation of bluetooth behind the scenes, which is a new style of load usage on Bluetooth, that it has messed up the way Bluetooth is managed in Yosemite. I'm guessing that Apple is aware of this and will fix it eventually. Until then, "Turn Off Wifi" seems to do the trick for me when I want to 'wakee up' my MacBook's bluetooth and get my external devices connected normally.