Multiple regression where only part of the sample completed the follow up survey

For my dissertation I trying to determine whether several emotions predict behavioural intention and behaviour. Participants were asked to complete two surveys, but approximately 1/4 only completed the first. I have completed the analysis for the intention questionnaire, but now need to run regressions to determine whether the emotions predict the behaviour.

I'm unsure how to do this when only part of the sample completed both parts. I'd appreciate any advice!