Bjorn Ulvaeus thinks that the ABBA comeback is an "immense risk." The 77-year-old musician began his ascent to global fame when he won the Eurovision Song Contest for Sweden with the song 'Waterloo' alongside ex-wife Agnetha Faltskog as well as Anni-Frid Lyngstad, and Benny Andersson back in 1974 and the four are set to reunite as digital avatars for a series of London concerts, and but Bjorn has been losing sleep over the comeback.