Most German past participles start with ge-: gemacht, gekauft, gespielt. Verbs with the suffix -ieren break this habit. They end in -t like any regular verb, but they never take ge-. So it is studiert, repariert, telefoniert and organisiert, never gestudiert or gerepariert.
English speakers trip over this for two reasons. First, the rule for ge- is drilled so early that it becomes automatic. Second, many of these verbs look like English words (studieren, akzeptieren, installieren), so learners treat them as ordinary regular verbs and add the prefix without thinking. The good news is that this group is very large and completely reliable, so one simple rule covers dozens of words.
The perfect is built as usual: a form of haben (or sometimes sein) plus the past participle at the end. For -ieren verbs you form the participle by swapping -en for -t: reparieren becomes repariert. Nothing goes in front.
The reason is stress. German only adds ge- when the verb is stressed on its first syllable, as in MA-chen and gemacht. The suffix -ieren always carries the stress (stu-DIE-ren, re-pa-RIE-ren), so there is no room for ge-. Verbs with inseparable prefixes such as be- and ver- behave the same way for the same reason.
Ich habe das Fahrrad repariert.
(I repaired the bike.)
Sie hat in Wien Medizin studiert.
(She studied medicine in Vienna.)
Gestern habe ich lange mit meiner Mutter telefoniert.
(Yesterday I talked on the phone with my mother for a long time.)
The words
Here are the verbs, grouped by topic. Every one of them forms its participle without ge-.
With anprobieren, the prefix an- stays in front, but the suffix still blocks ge-. The participle is anprobiert, not angeprobiert.
Hast du die Jacke schon anprobiert?
(Have you tried the jacket on yet?)
Reflexive verbs work the same way. Keep the reflexive pronoun and drop ge-: sich rasieren, sich konzentrieren, sich registrieren, sich immatrikulieren, sich amüsieren.
Wir haben uns auf der Party gut amüsiert.
(We had a great time at the party.)
The auxiliary is a separate question
Most of these verbs use haben, but passieren and explodieren take sein. The missing ge- stays the same either way.
Was ist denn passiert?
(What happened?)
Look-alikes
The rule is about the stressed suffix -ieren, not just the letters at the end. In frieren and verlieren, the -ieren is not a suffix. They are irregular verbs with the participles gefroren and verloren.
Im Winter haben wir an der Bushaltestelle oft gefroren.
(In winter we often froze at the bus stop.)
Summary
If a verb ends in the stressed suffix -ieren, build its participle by replacing -en with -t and add nothing in front: studiert, repariert, anprobiert. Choose haben or sein as you normally would, and don't be fooled by frieren and verlieren, which are not suffix verbs.