On Most Samsung remotes, the source button is on the top center area of the remote. Press it to select your input. (I believe you can press it multiple times to go through the inputs, or press it once and then move around with arrows.)
If you're not changing the input/source, then your description sounds mostly normal, except a few details: The Blu Ray and Roku should both be connected to the TV via HDMI. If you think the TV is connected to the Roku via Ethernet, then that's not going to work. For example if you removed the Roku HDMI cable in order to plug in the Blu Ray, either due to a lack of HDMI cables or a lack of HDMI ports, then you will have to do the reverse to get to the Roku. (You probably want an HDMI switch if you are out of ports on the TV. But look carefully, most TVs have 2 to 4 HDMI ports - sometimes in more than one place.)
But hopefully your Roku is still plugged in to a different HDMI port and all you need to do is use the source button.
If you have HDMI-CEC/Anynet+ enabled on the TV, then the TV may automagically switch inputs at times. This can save a button press or two when it works, but can be confusing when it doesn't - especially if it fails infrequently enough that you forget how to switch inputs yourself. If you find yourself getting confused by that, you can turn off Anynet+ on the TV (under settings) and always select the input yourself.
Oh, and it helps to press the home button on the Roku remote to make sure the Roku is awake. If the automatic power savings option on your Roku is enabled, then the TV should still switch to the Roku's input, but will show it as "no signal". This is perfectly normal, since there is indeed "no signal" when the Roku is sleeping, but can cause some people to think they are on the wrong input and switch to another one. If that's annoying you can turn it off, since a Roku sitting at the home screen uses little power anyway. That setting is at: Settings -> System -> Power -> Auto Power Savings.
